Acoustic intensity

Sound measurements
Sound pressure p, SPL
Particle velocity v, SVL
Particle displacement ξ
Sound intensity I, SIL
Sound power Pac
Sound power level SWL
Sound energy
Sound exposure E
Sound exposure level SEL
Sound energy density E
Sound energy flux q
Acoustic impedance Z
Speed of sound
Audio frequency AF

Sound intensity or acoustic intensity (I) is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.

Sound intensity is not the same physical quantity as related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.

Acoustic intensity

The intensity is the product of the sound pressure and the particle velocity

\vec{I} = p \vec{v}

Notice that both v and I are vectors, which means that both have a direction as well as a magnitude. The direction of the intensity is the average direction in which the energy is flowing. For instantaneous acoustic pressure pinst(t) and particle velocity v(t) the average acoustic intensity during time T is given by

I = \frac{1}{T} \int_{0}^{T}p_\mathrm{inst}(t) v(t)\,dt

The SI units of intensity are W/m2 (watts per square metre). For a plane progressive wave we have:

I = \frac{p^2}{Z} = Z v^2 = \xi^2 \omega^2 Z = \frac{a^2 Z}{\omega^2} = E c = \frac{P_{ac}}{A}

where:

Symbol Units Meaning
p pascals RMS sound pressure
f hertz frequency
ξ m, metres particle displacement
c m/s speed of sound
v m/s particle velocity
ω = 2πf radians/s angular frequency
ρ kg/m3 density of air
Z = c ρ N·s/m³ characteristic acoustic impedance
a m/s² particle acceleration
I W/m² sound intensity
E W·s/m³ sound energy density
Pac W, watts sound power or acoustic power
A m² area

Spatial expansion

For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:

I_r = \frac{P_{ac}}{A} = \frac{P_{ac}}{4 \pi r^2} \,

Here, Pac (upper case) is the sound power and A the surface area of a sphere of radius r. Thus the sound intensity decreases with 1/r2 the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r-distance law.

I \propto {p^2} \propto \dfrac{1}{r^2} \,

\dfrac{I_2}{I_1} = \dfrac \,

(W = watt), then instead of "dB SPL" we use "dB SIL". (SIL = sound intensity level).

References

External links

  • How many decibels is twice as loud? Sound level change and the respective factor of sound pressure or sound intensity
  • Acoustic Intensity
  • Conversion: Sound intensity level to sound intensity and vice versa
  • Ohm's law as acoustic equivalent - calculations
  • Relationships of acoustic quantities associated with a plane progressive acoustic sound wave - pdf
  • Table of Sound Levels - Corresponding Sound Intensity and Sound Pressure
  • What is Sound Intensity Measurement and Analysis?
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